Mesa is painful to bisect, I hope this can be improved

[Thomas Debesse]

Hi, for a long time now I wanted to make this observation in the hope
it can be improved:

Mesa is painfully hard to bisect because of many things.

The most common thing hindering bisect tasks is the fact some options
values are constantly swapped to `true` to `enabled` and things like
that. Even when there are some legacy-compatibility implemented to
recognize one for the other, Meson looks to only be able to process
the compatibility if and only if the build folder has been entirely
cleaned-up first. So bisecting requires to clean-up the build folder
on almost every bisecting step, and most of the bisecting steps also
require to modify the options themselves.

I wonder if there are some rooms to improve the bisecting experience:

- either by implementing some working method like not doing random
enable/true change one by one throughout the years but focusing at
porting all of them at once,
- either at making Meson accept one for the other in all cases.

Then comes the silent breaking of enabled features requiring another
feature to be enabled too to be working again, which also prevents to
bisect properly because the same configure and build commands for the
same feature may or may not produce a working installation given the
commit that is tested while bisecting, this has been reported there:

- https://gitlab.freedesktop.org/mesa/mesa/-/issues/12024

Then comes the usual renaming things like the `swrast` gallium driver
option being split into by `llvmpipe` and `softpipe`, we can't
probably avoid that, and I would be happy if in the future I and
others would only have to deal with that.

Do you you also experience similar problems when bisecting Mesa? Do
you also face similar problems but some I may have not listed ? I
focus myself on OpenGL on Xorg with GLX and on OpenCL (clover/rusticl)
so other people bisecting other things like GL ES, EGL, Vulkan or even
some drivers I'm not testing myself that much or not at all may
experience other things I'm not aware of but making bisecting uneasy
as well.

An example of thing I don't test that much is zink, so problems like
zink not being working while being installed given which commit is
built is definitely something that makes bisecting painful, but it
took me months to experience this problem myself only because I don't
test zink that much myself, but I guess some people experience similar
problems and pain with other features or components of Mesa. More info
about that zink problem can be found there, but this is just yet one
example among others:

- https://gitlab.freedesktop.org/mesa/mesa/-/issues/12024

While I don't bisect zink that much myself, when I had to bisect r300
to discover what Meson option I needed to make it work again, I
experienced the pain of having to rewrite Meson command line options
and to clean-up the build on various bisecting steps. So I expect
other people to experience similar problems I'm not aware of, only
because they bisect other things in Mesa I don't bisect myself.

Do you have any opinion on the topic and/or ideas to make bisecting Mesa easier?

-- 
Thomas “illwieckz” Debesse